N choose K in Scala

nchoosek.scala

case class Choice[A](chosen: List[A], unchosen: List[A])

def nChoose1[A](n: List[A]): Stream[Choice[A]] = n match {
  case Nil => Stream.empty[Choice[A]]
  case head :: tail => Choice(head :: Nil, tail) #:: nChoose1(tail).map(choice => choice.copy(unchosen = head :: choice.unchosen))
}

def nChooseK[A](n: List[A], k: Int): Stream[Choice[A]] = {
  if (k <= 0) Stream.empty[Choice[A]]
  else if (k == 1) nChoose1(n)
  else {
    val rec = nChooseK(n, k - 1)
    
    rec.flatMap { kminus1 =>
      nChoose1(kminus1.unchosen).map(one => kminus1.copy(chosen = kminus1.chosen ++ one.chosen, unchosen = kminus1.unchosen -- one.chosen))
    }
  }
}